1. Field of the Invention
The invention relates to a control device, more particularly to a control device for a motor unit of a food processor.
2. Description of the Related Art
In a conventional food processor, a cutting blade member is driven by a motor unit to rotate during a processing period when operating in a processing mode. However, rotation of the cutting blade member does not stop immediately when the conventional food processor is deactivated.
FIGS. 1 and 2 illustrate a conventional control circuit for an alternating current series motor 11 of a food processor (not shown). The motor 11 has two stator coil units 112, 113, and a rotor coil unit 111. The conventional control circuit includes a controller (not shown) operable in a processing mode and a braking mode, and first and second switches 13, 12 interconnecting electrically the stator coil units 112, 113 and the rotor coil unit 111. Each of the first and second switches 13, 12 includes a common node (C), a first switch node (NC) and a second switch node (NO). The first switch nodes (NC) of the first and second switches 13, 12 are connected to each other. The second switch node (NO) of the first switch 13 is connected electrically to one end of the stator coil unit 112 that is connected to one end of the rotor coil unit 111. The common node (C) of the first switch 13 is connected electrically to one end of the stator coil unit 113. The first switch node (NC) of the second switch 12 is connected electrically to the other end of the rotor coil unit 111. The common node (C) of the second switch 12 is connected electrically to the other end of the stator coil unit 113.
When the controller is operated in the processing mode, the first and second switches 13, 12 are controlled by the controller so as to connect the common node (C) to the first switch node (NC) in the first switch 13 and to connect the common node (C) to the second switch node (NO) in the second switch 12, as shown in FIG. 1, so that a first current path indicated by the arrows in FIG. 1 for an alternating current signal (AC) through the motor 11 is generated.
On the other hand, when the controller is operated in the braking mode, the first and second switches 13, 12 are controlled by the controller so as to connect the common node (C) to the second switch node (NO) in the first switch 13 and to connect the common node (C) to the first switch node (NC) in the second switch 12, as shown in FIG. 2, so that a second current path indicated by the arrows in FIG. 2 for a residual power signal in the stator coil unit 113 through the rotor coil unit 111 is generated.
Since a voltage supplied to the rotor coil unit 111 in the second current path has a polarity opposite to that in the first current path, sparks or vibration may occur during the transition from the processing mode to the braking mode.
Furthermore, the residual voltage signal in the stator coil unit 113 is insufficient to stop rotation of the motor 11 within a short amount of time. Therefore, safety during a relatively long braking period cannot be ensured.